fix(pallet-multi-asset-bounties): use non-destructive read in calculate_payout()

[dhirajs0]

Description

calculate_payout() in pallet-multi-asset-bounties uses ChildBountiesValuePerParent::take() — a destructive read that deletes the storage entry — instead of get(). Since calculate_payout() is called from multiple code paths, the take() causes incorrect behavior on subsequent calls.

calculate_payout() is called from two places:

1. do_process_payout_payment() (lib.rs:1736) — invoked by award_bounty() and retry_payment()
2. do_check_payout_payment_status() (lib.rs:1771) — invoked by check_status() on payment success

When a parent bounty with child bounties is awarded:

• The first call (from award_bounty()) reads ChildBountiesValuePerParent via take(), correctly computing parent_value - children_value, but deletes the storage entry as a side effect.
• The second call (from check_status() on success) reads the now-deleted storage, gets 0, and emits BountyPayoutProcessed with value: parent_value instead of the correct value: parent_value - children_value.

Additionally, if a non-synchronous Paymaster implementation is used where check_payment() can return Failure, the retry_payment() path would call calculate_payout() again on the deleted storage, attempting to pay the full parent value instead of the reduced amount.

Integration

No integration changes required for downstream projects. This is a bugfix internal to pallet-multi-asset-bounties with no changes to public APIs, storage layout, or trait definitions.

Review Notes

Three changes were made:

1. calculate_payout() — take() replaced with get()

- let children_value = ChildBountiesValuePerParent::<T, I>::take(parent_bounty_id);
+ let children_value = ChildBountiesValuePerParent::<T, I>::get(parent_bounty_id);

This makes calculate_payout() idempotent — safe to call multiple times for the same bounty.

2. remove_bounty() — explicit storage cleanup added

  None => {
      Bounties::<T, I>::remove(parent_bounty_id);
      ChildBountiesPerParent::<T, I>::remove(parent_bounty_id);
      TotalChildBountiesPerParent::<T, I>::remove(parent_bounty_id);
-     debug_assert!(ChildBountiesValuePerParent::<T, I>::get(parent_bounty_id).is_zero());
+     ChildBountiesValuePerParent::<T, I>::remove(parent_bounty_id);
  },

The debug_assert! was removed because it was not a true invariant — it only passed because take() had already deleted the value. When child bounties are paid out, ChildBountiesValuePerParent remains non-zero until parent bounty cleanup.

3. Test updated

Added an event assertion to the existing check_status_works test to verify BountyPayoutProcessed emits the correct net payout value (parent_value - child_value) instead of the full parent value. This assertion fails with take() and passes with get().

Impact

• Current deployments (KAH, PAH): Both use LocalPay where check_payment() always returns Success. The retry/lock path is unreachable, but the BountyPayoutProcessed event emits an incorrect payout value for parent bounties with child bounties.
• Future deployments: If the pallet is configured with an async Paymaster (e.g., XCM-based) where check_payment() can return Failure, the retry_payment() path would compute a wrong payout amount, potentially leading to permanent fund lock with no recovery path (since close_bounty() rejects PayoutAttempted status).

[dhirajs0]

/cmd fmt

[dhirajs0]

/cmd prdoc

[github-actions]

Review required! Latest push from author must always be reviewed

[paritytech-release-backport-bot]

Created backport PR for stable2512:

• [stable2512] Backport #11425 #11622 with remaining conflicts!

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-11425-to-stable2512
git worktree add --checkout .worktree/backport-11425-to-stable2512 backport-11425-to-stable2512
cd .worktree/backport-11425-to-stable2512
git reset --hard HEAD^
git cherry-pick -x 5fbeadde2a12aa6c3bd97b64a82748e715ae7813
git push --force-with-lease

[paritytech-release-backport-bot]

Successfully created backport PR for stable2603:

• [stable2603] Backport #11425 #11623